I'm not sure how you can think that Apple has a monopoly in digital music. Are you sure you know what the word monopoly means? It doesn't mean having a popular product or service, it means having virtually the only source for a particular commodity. Obviously, digital music is available from many, many sources. Where's the monopoly? Now, like any business, I'm sure Apple wishes it could have a monopoly in digital music, but it doesn't and it never will. It's just not possible.

Back in November, Apple has 87% of hard drive based MP3 players in north america. As far as I know, the ONLY way to legally buy music for this iPods (again, 87% of all market share), is from iTunes Music Store and *only* iTunes Music Store. (there's Real's hacked workaround to their music work in iPod, but how it's "legal" depends how you interpret the law... as apple has taken legal actions against then because they didn't license the technology from Apple, even if Real tried to).
I do consider this monopoly. You may think different.

You are correct that the only Online Music Store that you can buy music from for the iPod is the iTunes Music Store, but the iPod will also let you put MP3 and other file types on it. So you do not have to buy music from the iTunes Music Store in order to put music on your iPod. In fact you can even set iTunes (the application, not the store) to not even show the Music Store. So although they may control over what music you buy online for the iPod, they are not making you only use it.

On a side note. I believe that Apple will support WMA-protected files on the iPod, which will allow you to buy music from other online music stores, once Microsoft makes those file types work on the Mac. I believe that the last thing Apple wants to do is say that the iPod can do WMA-protected files but it is Windows only.

"As far as I know, the ONLY way to legally buy music for this iPods is from iTunes Music Store and *only* iTunes Music Store. (...) I do consider this monopoly. You may think different."

You're completely wrong.
As Anonymous already said, you can play .mp3s on your iPod, no matter where they came from. Heck, you can also buy CDs from Amazon or a music store in your town (what a concept) and rip it to the iPod --- legally.
So, actually, I think different.

mmm emusic comes to mind.. no problems putting that on an ipod.. i think what you mean is the only legal music you may download and put on an iPod that contains DRM is from the iTunes music store.. but there are plenty of sites that offer music in one of the dozen of formats iPods can play